Progressive apoptosis resistance prior to senescence and control by the anti-apoptotic protein BCL-xL

Mitochondria in cell senescence: A Friend or Foe?

Cell senescence denotes cell growth arrest in response to continuous replication or stresses damaging DNA or mitochondria. Mounting research suggests that cell senescence attributes to aging-associated failing organ function and diseases. Conversely, it participates in embryonic tissue maturation, wound healing, tissue regeneration, and tumor suppression. The acute or chronic properties and microenvironment may explain the double faces of senescence. Senescent cells display unique characteristics. In particular, its mitochondria become elongated with altered metabolomes and dynamics. Accordingly, mitochondria reform their function to produce more reactive oxygen species at the cost of low ATP production. Meanwhile, destructed mitochondrial unfolded protein responses further break the delicate proteostasis fostering mitochondrial dysfunction. Additionally, the release of mitochondrial damage-associated molecular patterns, mitochondrial Ca²⁺ overload, and altered NAD⁺ level intertwine other cellular organelle strengthening senescence. These findings further intrigue researchers to develop anti-senescence interventions. Applying mitochondrial-targeted antioxidants reduces cell senescence and mitigates aging by restoring mitochondrial function and attenuating oxidative stress. Metformin and caloric restriction also manifest senescent rescuing effects by increasing mitochondria efficiency and alleviating oxidative damage. On the other hand, Bcl2 family protein inhibitors eradicate senescent cells by inducing apoptosis to facilitate cancer chemotherapy. This review describes the different aspects of mitochondrial changes in senescence and highlights the recent progress of some anti-senescence strategies.

Targeting Mitochondria to Control Ageing and Senescence

Ageing is accompanied by a progressive impairment of cellular function and a systemic deterioration of tissues and organs, resulting in increased vulnerability to multiple diseases. Here, we review the interplay between two hallmarks of ageing, namely, mitochondrial dysfunction and cellular senescence. The targeting of specific mitochondrial features in senescent cells has the potential of delaying or even reverting the ageing process. A deeper and more comprehensive understanding of mitochondrial biology in senescent cells is necessary to effectively face this challenge. Here, we discuss the main alterations in mitochondrial functions and structure in both ageing and cellular senescence, highlighting the differences and similarities between the two processes. Moreover, we describe the treatments available to target these pathways and speculate on possible future directions of anti-ageing and anti-senescence therapies targeting mitochondria.

Bie-Jia-Ruan-Mai-Tang, a Chinese Medicine Formula, Inhibits Retinal Neovascularization in Diabetic Mice Through Inducing the Apoptosis of Retinal Vascular Endothelial Cells

Proliferative diabetic retinopathy (PDR) is one of the main complications of diabetes, mainly caused by the aberrant proliferation of retinal vascular endothelial cells and the formation of new blood vessels. Traditional Chinese medicines possess great potential in the prevention and treatment of PDR. Bie-Jia-Ruan-Mai-Tang (BJ), a Chinese medicine formula, has a good therapeutic effect on PDR clinically; however, the mechanism of action involved remains unclear. Therefore, we investigated the effect of BJ on PDR through in vitro and in vivo experiments. A diabetic mouse model with PDR was established by feeding a high-fat–high-glucose diet combined with an intraperitoneal injection of streptozotocin (STZ), while high-glucose-exposed human retinal capillary endothelial cells (HRCECs) were employed to mimic PDR in vitro. The in vivo experiments indicated that BJ inhibited the formation of acellular capillaries, decreased the expression of VEGF, and increased the level of ZO-1 in diabetic mice retina. In vitro experiments showed that high glucose significantly promoted cell viability and proliferation. However, BJ inhibited cell proliferation by cycle arrest in the S phase, thus leading to apoptosis; it also increased the production of ROS, decreased the mitochondrial membrane potential, reduced the ATP production, and also reduced the expressions of p-PI3K, p-AKT, and Bcl-xL, but increased the expressions of Bax and p-NF-κB. These results suggest that BJ induces the apoptosis of HRCECs exposed to high glucose through activating the mitochondrial death pathway by decreasing the PI3K/AKT signaling and increasing the NF-κB signaling to inhibit the formation of acellular capillaries in the retina, thus impeding the development of PDR.

BAK plays a key role in A-1331852-induced apoptosis in senescent chondrocytes

Osteoarthritis occurs when the number of senescent chondrocytes in the joints reaches an intolerable level. The purpose of our study was to explore the therapeutic effect and mechanism of action of A-1331852 in osteoarthritis. Doxorubicin and etoposide were used to induce cell senescence as determined by the cessation of cell proliferation, augmented senescence-associated beta-galactosidase (SA-β-Gal) staining, and increased p53 expression levels. The CCK-8 cytotoxicity assay and SA-β-Gal staining demonstrated that Bcl-xL inhibitors could selectively remove senescent chondrocytes without damaging healthy chondrocytes. A-1331852 induced caspase-dependent death of senescent chondrocytes with decreased mitochondrial membrane potential, nuclear concentration, plasma membrane rupture, and PARP cleavage. Most importantly, A-1331852 upregulated BAK expression levels, indicating that BAK plays a key role in the A-1331852-induced apoptosis of senescent chondrocytes. Live-cell fluorescence resonance energy transfer showed that A-1331852 detached the binding of Bcl-xL to BAK and promoted the oligomerization of BAK on the mitochondrial membrane. In conclusion, this study provides the first evidence that A-1331852 selectively promotes apoptosis in senescent chondrocytes by interfering with the interaction between Bcl-xL and BAK.

Why Senescent Cells Are Resistant to Apoptosis: An Insight for Senolytic Development

Cellular senescence is a process that leads to a state of irreversible cell growth arrest induced by a variety of intrinsic and extrinsic stresses. Senescent cells (SnCs) accumulate with age and have been implicated in various age-related diseases in part via expressing the senescence-associated secretory phenotype. Elimination of SnCs has the potential to delay aging, treat age-related diseases and extend healthspan. However, once cells becoming senescent, they are more resistant to apoptotic stimuli. Senolytics can selectively eliminate SnCs by targeting the SnC anti-apoptotic pathways (SCAPs). They have been developed as a novel pharmacological strategy to treat various age-related diseases. However, the heterogeneity of the SnCs indicates that SnCs depend on different proteins or pathways for their survival. Thus, a better understanding of the underlying mechanisms for apoptotic resistance of SnCs will provide new molecular targets for the development of cell-specific or broad-spectrum therapeutics to clear SnCs. In this review, we discussed the latest research progresses and challenge in senolytic development, described the significance of regulation of senescence and apoptosis in aging, and systematically summarized the SCAPs involved in the apoptotic resistance in SnCs.

Cellular mechanisms underlying response and resistance to CDK4/6 inhibitors in the treatment of hormone receptor-positive breast cancer

Pharmacological inhibitors of cyclin-dependent kinases 4 and 6 (CDK4/6) are now an established standard of care for patients with advanced hormone receptor-positive breast cancer. The canonical mechanism underlying CDK4/6 inhibitor activity is the suppression of phosphorylation of the retinoblastoma tumor suppressor protein, which serves to prevent cancer cell proliferation. Recent data suggest that these agents induce other diverse effects within both tumor and stromal compartments, which serve to explain aspects of their clinical activity. Here, we review these phenomena and discuss how they might be leveraged in the development of novel CDK4/6 inhibitor-containing combination treatments. We also briefly review the various known mechanisms of acquired resistance in the clinical setting.

Senescent skeletal cells cross-talk with synovial cells plays a key role in the pathogenesis of osteoarthritis

Osteoarthritis (OA) has been recognized as an age-related degenerative disease commonly seen in the elderly that affects the whole “organ” including cartilage, subchondral bone, synovium, and muscles. An increasing number of studies have suggested that the accumulation of senescent cells triggering by various stresses in the local joint contributes to the pathogenesis of age-related diseases including OA. In this review, we mainly focus on the role of the senescent skeletal cells (chondrocytes, osteoblasts, osteoclasts, osteocyte, and muscle cells) in initiating the development and progression of OA alone or through cross-talk with the macrophages/synovial cells. Accordingly, we summarize the current OA-targeted therapies based on the abovementioned theory, e.g., by eliminating senescent skeletal cells and/or inhibiting the senescence-associated secretory phenotype (SASP) that drives senescence. Furthermore, the existing animal models for the study of OA from the perspective of senescence are highlighted to fill the gap between basic research and clinical applications. Overall, in this review, we systematically assess the current understanding of cellular senescence in OA, which in turn might shed light on the stratified OA treatments.

Microbiome and Human Aging: Probiotic and Prebiotic Potentials in Longevity, Skin Health and Cellular Senescence

The role of the microbiome in human aging is important: the microbiome directly impacts aging through the gastrointestinal system. However, the microbial impact on skin has yet to be fully understood. For example, cellular senescence is an intrinsic aging process that has been recently associated with microbial imbalance. With age, cells become senescent in response to stress wherein they undergo irreversible growth arrest while maintaining high metabolic activity. An accumulation of senescent cells has been linked to various aging and chronic pathologies due to an overexpression of the senescence-associated secretory phenotype (SASP) comprised of proinflammatory cytokines, chemokines, growth factors, proteases, lipids and extracellular matrix components. In particular, dermatological disorders may be promoted by senescence as the skin is a common site of accumulation. The gut microbiota influences cellular senescence and skin disruption through the gut-skin axis and secretion of microbial metabolites. Metabolomics can be used to identify and quantify metabolites involved in senescence. Moreover, novel anti-senescent therapeutics are warranted given the poor safety profiles of current pharmaceutical drugs. Probiotics and prebiotics may be effective alternatives, considering the relationship between the microbiome and healthy aging. However, further research on gut composition under a senescent status is needed to develop immunomodulatory therapies.

Long-term cardiovascular complications following sepsis: is senescence the missing link?

Among the long-term consequences of sepsis (also termed “post-sepsis syndrome”) the increased risk of unexplained cardiovascular complications, such as myocardial infarction, acute heart failure or stroke, is one of the emerging specific health concerns. The vascular accelerated ageing also named premature senescence is a potential mechanism contributing to atherothrombosis, consequently leading to cardiovascular events. Indeed, vascular senescence-associated major adverse cardiovascular events (MACE) are a potential feature in sepsis survivors and of the elderly at cardiovascular risk. In these patients, accelerated vascular senescence could be one of the potential facilitating mechanisms. This review will focus on premature senescence in sepsis regardless of age. It will highlight and refine the potential relationships between sepsis and accelerated vascular senescence. In particular, key cellular mechanisms contributing to cardiovascular events in post-sepsis syndrome will be highlighted, and potential therapeutic strategies to reduce the cardiovascular risk will be further discussed.

With improved management of patients, sepsis survivors are increasing each year.

Early cardiovascular complications, of yet undeciphered mechanisms, are an emerging health issue in post-sepsis syndrome.

Premature senescence of endothelium and vascular tissue is proven an accelerated process of atherogenesis in young septic rats.

An increasing body of clinical evidence point at endothelial senescence in the initiation and development of atherosclerosis.

Prevention of premature senescence by senotherapy and cardiological follow-up could improve long-term septic patients’ outcomes.

Skin Aging, Cellular Senescence and Natural Polyphenols

The skin, being the barrier organ of the body, is constitutively exposed to various stimuli impacting its morphology and function. Senescent cells have been found to accumulate with age and may contribute to age-related skin changes and pathologies. Natural polyphenols exert many health benefits, including ameliorative effects on skin aging. By affecting molecular pathways of senescence, polyphenols are able to prevent or delay the senescence formation and, consequently, avoid or ameliorate aging and age-associated pathologies of the skin. This review aims to provide an overview of the current state of knowledge in skin aging and cellular senescence, and to summarize the recent in vitro studies related to the anti-senescent mechanisms of natural polyphenols carried out on keratinocytes, melanocytes and fibroblasts. Aged skin in the context of the COVID-19 pandemic will be also discussed.

The interplay between apoptosis and cellular senescence: Bcl-2 family proteins as targets for cancer therapy

Cell death by apoptosis and permanent cell cycle arrest by senescence serve as barriers to the development of cancer. Chemotherapeutic agents not only induce apoptosis, they can also induce senescence known as therapy-induced senescence (TIS). There are, however, controversies whether TIS improves or worsens therapeutic outcome. Unlike apoptosis, which permanently removes cancer cells, senescent cells are metabolically active, and can contribute to tumor progression and relapse. If senescent cells are not cleared by the immune system or if cancer cells escape senescence, they may acquire resistance to apoptotic stimuli and become highly aggressive. Thus, there have been significant efforts in developing senolytics, drugs that target these pro-survival molecules to eliminate senescent cells. The anti-apoptotic Bcl-2 family proteins not only protect against cell death by apoptosis, but they also allow senescent cells to survive. While combining senolytics with chemotherapeutic drugs is an attractive approach, there are also limitations. Moreover, members of the Bcl-2 family have distinct effects on apoptosis and senescence. The purpose of this review article is to discuss recent literatures on how members of the Bcl-2 family orchestrate the interplay between apoptosis and senescence, and the challenges and progress in targeting these Bcl-2 family proteins for cancer therapy.

Biological functions of therapy-induced senescence in cancer

Therapy-induced cellular senescence is a state of stable growth arrest induced by common cancer treatments such as chemotherapy and radiation. In an oncogenic context, therapy-induced senescence can have different consequences. By blocking cellular proliferation and by facilitating immune cell infiltration, it functions as tumor suppressive mechanism. By fueling the proliferation of bystander cells and facilitating metastasis, it acts as a tumor promoting factor. This dual role is mainly attributed to the differential expression and secretion of a set of pro-inflammatory cytokines and tissue remodeling factors, collectively known as the Senescence-Associated Secretory Phenotype (SASP). Here, we describe cell-autonomous and non-cell-autonomous mechanisms that senescent cells activate in response to chemotherapy and radiation leading to tumor suppression and tumor promotion. We present the current state of knowledge on the stimuli that affect the activation of these opposing mechanisms and the effect of senescent cells on their micro-environment eg. by regulating the functions of immune cells in tumor clearance as well as strategies to eliminate senescent tumor cells before exerting their deleterious side-effects.

miR-30c-1 encourages human corneal endothelial cells to regenerate through ameliorating senescence

In the present study, we studied the role of microRNA-30c-1 (miR-30c-1) on transforming growth factor beta1 (TGF-β1)-induced senescence of hCECs. hCECs were transfected by miR-30c-1 and treated with TGF-β1 to assess the inhibitory effect of miR-30c-1 on TGF-β1-induced senescence. Cell viability and proliferation rate in miR-30c-1-transfected cells was elevated compared with control. Cell cycle analysis revealed that cell abundance in S phase was elevated in miR-30c-1-treated cells compared with control. TGF-β1 increased the senescence of hCECs; however, this was ameliorated by miR-30c-1. TGF-β1 increased the size of hCECs, the ratio of senescence-associated beta-galactosidase-stained cells, secretion of senescence-associated secretory phenotype factors, the oxidative stress, and arrested the cell cycle, all of which were ameliorated by miR-30c-1 treatment. miR-30c-1 also suppressed a TGF-β1-induced depolarization of mitochondrial membrane potential and a TGF-β1 stimulated increase in levels of cleaved poly (ADP-ribose) polymerase (PARP), cleaved caspase 3, and microtubule-associated proteins 1A/1B light chain 3B II. In conclusion, miR-30c-1 promoted the proliferation of hCECs through ameliorating the TGF- β1-induced senescence of hCECs and reducing cell death of hCECs. Thus, miR-30c-1 may be a therapeutic target for hCECs regeneration.

Bcl-xL as a Modulator of Senescence and Aging

Many features of aging result from the incapacity of cells to adapt to stress conditions. When cells are overwhelmed by stress, they can undergo senescence to avoid unrestricted growth of damaged cells. Recent findings have proven that cellular senescence is more than that. A specific grade of senescence promotes embryo development, tissue remodeling and wound healing. However, constant stresses and a weakening immune system can lead to senescence chronicity with aging. The accumulation of senescent cells is directly related to tissue dysfunction and age-related pathologies. Centenarians, the most aged individuals, should accumulate senescent cells and suffer from their deleterious effects, however, they enjoy a compression of morbidity. We have shown that they overexpress B-cell lymphoma-extra large (Bcl-xL). Bcl-xL could avoid an excessive burden of senescent cells through the regulation of intrinsic apoptosis, mitochondrial bioenergetics and oxidative stress. On the other hand, Bcl-xL maintains a fully functional immune system that ensures an efficient clearance of senescent cells. Moreover, there is a paradox, as inhibitors of Bcl-xL have been employed as senolytic agents, which have been shown to protect from aging in animal models. In this review, we aim to discuss how Bcl-xL could modulate senescence-associated harmful effects in centenarians, protecting them from the burden of accumulation of senescent cells.

Apoptosis, the only cell death pathway that can be measured in human diploid dermal fibroblasts following lethal UVB irradiation

Ultraviolet radiation (UVR) is a major environmental genotoxic agent. In skin, it can lead to the formation of mutagenic DNA damage. Several mechanisms are in place to prevent the conversion of these DNA damage into skin cancer-driver mutations. An important mutation prevention mechanism is the programmed cell death, which can safely dispose of the damaged cells. Apoptosis is the most studied and best characterised programmed cell death, but an increasing amount of new cell death pathways are emerging. Using different pharmacological cell death inhibitors and antioxidants, we have evaluated the implication of apoptosis, necroptosis, ferroptosis and parthanatos in UVB-induced cell death in human diploid dermal fibroblasts. Our results show that apoptosis is the only known cell death mechanism induced by UVB irradiation in fibroblasts. We also showed that lethal UVB irradiation induces a PARP-dependent drastic loss of cellular metabolic activity caused by an overused of NAD+.

Bcl-XL: A multifunctional anti-apoptotic protein

B-cell lymphoma-extra large (Bcl-X_L) is one of the anti-apoptotic proteins of the Bcl-2 family that is localized in the mitochondria. Bcl-X_L is one of the key regulators of apoptosis that can also regulate other important cellular functions. Bcl-X_L is overexpressed in many cancers, and its inhibitors have shown good therapeutic effects. Bcl-X_L interacts with Beclin 1, a key factor regulating autophagy. Bcl-X_L is essential for the survival of neurons and plays protective roles in neuronal injuries. It can promote the growth of neurons and the correct formation of neural networks, enhance synaptic plasticity, and control neurotoxicity. Bcl-X_L can also promote the transport of Ca²⁺ to mitochondria, increase the production of ATP, and improve metabolic efficiency. In addition, targeting Bcl-X_L has shown potential value in autoimmune diseases and aging. In this review, we summarize the functions of Bcl-X_L in cancer, autophagy, Ca²⁺ signaling, neuroprotection, neuronal growth and synaptic plasticity, energy metabolism, immunity, and senescence as revealed by investigations conducted in the past 10 years. Moreover, we list some inhibitors that have been developed based on the functions of Bcl-X_L.

miR-125b Upregulates miR-34a and Sequentially Activates Stress Adaption and Cell Death Mechanisms in Multiple Myeloma

miR-125b, ubiquitously expressed and frequently dysregulated in several tumors, has gained special interest in the field of cancer research, displaying either oncogenic or oncosuppressor potential based on tumor type. We have previously demonstrated its tumor-suppressive role in multiple myeloma (MM), but the analysis of molecular mechanisms needs additional investigation. The purpose of this study was to explore the effects of miR-125b and its chemically modified analogs in modulating cell viability and cancer-associated molecular pathways, also focusing on the functional aspects of stress adaptation (autophagy and senescence), as well as programmed cell death (apoptosis). Based on the well-known low microRNA (miRNA) stability in therapeutic application, we designed chemically modified miR-125b mimics, laying the bases for their subsequent investigation in in vivo models. Our study clearly confirmed an oncosuppressive function depending on the repression of multiple targets, and it allowed the identification, for the first time, of miR-125b-dependent miR-34a stimulation as a possible consequence of the inhibitory role on the interleukin-6 receptor (IL-6R)/signal transducer and activator of transcription 3 (STAT3)/miR-34a feedback loop. Moreover, we identified a pattern of miR-125b-co-regulated miRNAs, shedding light on possible new players of anti-MM activity. Finally, functional studies also revealed a sequential activation of senescence, autophagy, and apoptosis, thus indicating, for the first two processes, an early cytoprotective and inhibitory role from apoptosis activation.

Low DNA damage in peripheral lymphocytes of healthy elderly individuals with inverted CD4:CD8 ratio

The aim of this study was to evaluate the DNA damage in peripheral lymphocytes and the frequencies of CD8+ T cells expressing CD25, CD28 and CD45ro in aged individuals with inverted CD4:CD8 ratio. Blood samples of elderly individuals (aged >65) with normal CD4:CD8 ratio (n = 8) and inverted CD4:CD8 ratio (n = 8) were collected to identify the expression of CD25+, CD28+ and CD45ro+ in CD8+ T cells. DNA damage index was evaluated by the alkaline comet assay which was performed in lymphocytes treated with different concentrations of methyl methanesulfonate (MMS) (control non-treatment, 2 × 10^-5 M, 4 × 10^-5 M) for 1, 2 or 24 h. Elderly individuals with inverted CD4:CD8 ratio presented low frequency of CD8+ CD28+. Moreover, low DNA damage was observed in lymphocytes of elderly with inverted CD4:CD8 ratio in different doses of MMS. Aged individuals with inverted CD4:CD8 ratio presented lower DNA damage events in peripheral lymphocytes, suggesting a resistance for cell death in T cells of individuals with immune risk profile.

d-galactose induces premature senescence of lens epithelial cells by disturbing autophagy flux and mitochondrial functions

Cataract is the leading cause of blindness with an estimated 16 million people affected worldwide. d-galactose (d-gal) is a reducing sugar that widely distributed in foodstuffs, and studies show that d-gal could promote cataract formation by damaging nature lens epithelial cells (LECs). However, the underlying mechanism is unclear. In our present study, d-gal resulted in premature senescence of LECs, which was confirmed by determining the β-galactosidase activity, cell proliferative potential and cell cycle distribution, though apoptosis of LECs was not observed. We also verified that d-gal induced the impairment of autophagy flux by measuring the expression of LC3II and P62. Meanwhile, we found that d-gal induced mitochondrial dysfunctions of LECs through increasing reactive oxygen species (ROS), reducing ATP synthesis and mitochondrial potential (MMP), enhancing the concentration of cytoplasm Ca²⁺ and permeability transition pore (mPTP) opening. Metformin, as a potential anti-aging agent, suppressed the senescence of LECs by restoring autophagy flux and mitochondria functions. Nevertheless, the antioxidant N-acetylcysteine (NAC) scavenged ROS significantly but was not efficient in preventing LECs from premature senescence. Our data suggests that restoring autophagy activity and improving mitochondrial functions may be a potential strategy for the prevention of LECs senescence-related cataract.

Naked mole rats can undergo developmental, oncogene-induced and DNA damage-induced cellular senescence

Cellular senescence is an important anticancer mechanism that restricts proliferation of damaged or premalignant cells. Cellular senescence also plays an important role in tissue remodeling during development. However, there is a trade-off associated with cellular senescence as senescent cells contribute to aging pathologies. The naked mole rat (NMR) (Heterocephalus glaber) is the longest-lived rodent that is resistant to a variety of age-related diseases. Remarkably, NMRs do not show aging phenotypes until very late stages of their lives. Here, we tested whether NMR cells undergo cellular senescence. We report that the NMR displays developmentally programmed cellular senescence in multiple tissues, including nail bed, skin dermis, hair follicle, and nasopharyngeal cavity. NMR cells also underwent cellular senescence when transfected with oncogenic Ras. In addition, cellular senescence was detected in NMR embryonic and skin fibroblasts subjected to γ-irradiation (IR). However, NMR cells required a higher dose of IR for induction of cellular senescence, and NMR fibroblasts were resistant to IR-induced apoptosis. Gene expression analyses of senescence-related changes demonstrated that, similar to mice, NMR cells up-regulated senescence-associated secretory phenotype genes but displayed more profound down-regulation of DNA metabolism, transcription, and translation than mouse cells. We conclude that the NMR displays the same types of cellular senescence found in a short-lived rodent.

Influence of a pre-stimulation with chronic low-dose UVB on stress response mechanisms in human skin fibroblasts

Exposure to solar ultraviolet type B (UVB), through the induction of cyclobutane pyrimidine dimer (CPD), is the major risk factor for cutaneous cancer. Cells respond to UV-induced CPD by triggering the DNA damage response (DDR) responsible for signaling DNA repair, programmed cell death and cell cycle arrest. Underlying mechanisms implicated in the DDR have been extensively studied using single acute UVB irradiation. However, little is known concerning the consequences of chronic low-dose of UVB (CLUV) on the DDR. Thus, we have investigated the effect of a CLUV pre-stimulation on the different stress response pathways. We found that CLUV pre-stimulation enhances CPD repair capacity and leads to a cell cycle delay but leave residual unrepaired CPD. We further analyzed the consequence of the CLUV regimen on general gene and protein expression. We found that CLUV treatment influences biological processes related to the response to stress at the transcriptomic and proteomic levels. This overview study represents the first demonstration that human cells respond to chronic UV irradiation by modulating their genotoxic stress response mechanisms.

Faster DNA Repair of Ultraviolet-Induced Cyclobutane Pyrimidine Dimers and Lower Sensitivity to Apoptosis in Human Corneal Epithelial Cells than in Epidermal Keratinocytes

Absorption of UV rays by DNA generates the formation of mutagenic cyclobutane pyrimidine dimers (CPD) and pyrimidine (6-4) pyrimidone photoproducts (6-4PP). These damages are the major cause of skin cancer because in turn, they can lead to signature UV mutations. The eye is exposed to UV light, but the cornea is orders of magnitude less prone to UV-induced cancer. In an attempt to shed light on this paradox, we compared cells of the corneal epithelium and the epidermis for UVB-induced DNA damage frequency, repair and cell death sensitivity. We found similar CPD levels but a 4-time faster UVB-induced CPD, but not 6-4PP, repair and lower UV-induced apoptosis sensitivity in corneal epithelial cells than epidermal. We then investigated levels of DDB2, a UV-induced DNA damage recognition protein mostly impacting CPD repair, XPC, essential for the repair of both CPD and 6-4PP and p53 a protein upstream of the genotoxic stress response. We found more DDB2, XPC and p53 in corneal epithelial cells than in epidermal cells. According to our results analyzing the protein stability of DDB2 and XPC, the higher level of DDB2 and XPC in corneal epithelial cells is most likely due to an increased stability of the protein. Taken together, our results show that corneal epithelial cells have a better efficiency to repair UV-induced mutagenic CPD. On the other hand, they are less prone to UV-induced apoptosis, which could be related to the fact that since the repair is more efficient in the HCEC, the need to eliminate highly damaged cells by apoptosis is reduced.

Whole-Body Induced Cell Turnover: A Proposed Intervention for Age-Related Damage and Associated Pathology

In both biomedicine in general and biomedical gerontology in particular, cell replacement therapy is traditionally proposed as an intervention for cell loss. This article presents a proposed intervention-whole-body induced cell turnover (WICT)-for use in biomedical gerontology that combines cell replacement therapy with a second therapeutic component (targeted cell ablation) so as to broaden the therapeutic utility of cell therapies and increase the categories of age-related damage that are amenable to cell-based interventions. In particular, WICT may allow cell therapies to serve as an intervention for accumulated cellular and intracellular damage, such as telomere depletion, genomic DNA and mitochondrial DNA damage and mutations, replicative senescence, functionally deleterious age-related changes in gene expression, accumulated cellular and intracellular aggregates, and functionally deleterious posttranslationally modified gene products. WICT consists of the gradual ablation and subsequent replacement of a patient's entire set of constituent cells gradually over the course of their adult life span through the quantitative and qualitative coordination of targeted cell ablation with exogenous cell administration. The aim is to remove age-associated cellular and intracellular damage present in the patient's endogenous cells. In this study, we outline the underlying techniques and technologies by which WICT can be mediated, describe the mechanisms by which it can serve to negate or prevent age-related cellular and intracellular damage, explicate the unique therapeutic components and utilities that distinguish it as a distinct type of cell-based intervention for use in biomedical gerontology, and address potential complications associated with the therapy.

Programmed cell death in aging

Programmed cell death (PCD) pathways, including apoptosis and regulated necrosis, are required for normal cell turnover and tissue homeostasis. Mis-regulation of PCD is increasingly implicated in aging and aging-related disease. During aging the cell turnover rate declines for several highly-mitotic tissues. Aging-associated disruptions in systemic and inter-cell signaling combined with cell-autonomous damage and mitochondrial malfunction result in increased PCD in some cell types, and decreased PCD in other cell types. Increased PCD during aging is implicated in immune system decline, skeletal muscle wasting (sarcopenia), loss of cells in the heart, and neurodegenerative disease. In contrast, cancer cells and senescent cells are resistant to PCD, enabling them to increase in abundance during aging. PCD pathways limit life span in fungi, but whether PCD pathways normally limit adult metazoan life span is not yet clear. PCD is regulated by a balance of negative and positive factors, including the mitochondria, which are particularly subject to aging-associated malfunction.

An Old Flame Never Dies: Survivin in Cancer and Cellular Senescence

Survivin (BIRC5) is highly expressed in the vast majority of human cancers and is associated with chemotherapy resistance, increased tumor recurrence and shortened patient survival, making it an attractive therapeutic target. Initially identified as an inhibitor of apoptosis protein, it also plays a major role in the regulation of cell division. As such, it acts as a subunit of the chromosomal passenger complex, composed of the mitotic kinase aurora B, borealin and inner centromere protein, and is essential for proper chromosome segregation and cytokinesis. For both biological functions, interaction of survivin's nuclear export signal with the nuclear export receptor chromosome region maintenance 1 is absolutely essential. The timely orchestration of survivin's wide protein interaction repertoire is further modulated by different posttranslational modifications occurring in a cell-cycle-dependent manner. Recent data furthermore indicate additional roles of survivin in the DNA damage response, contributing to therapy resistance, yet the underlying molecular details are still not completely resolved. This also holds true for a potential involvement of survivin in senescence regulation. An age-related accumulation of survivin probably contributes to the apoptosis resistance observed in aged as well as in senescent cells, while it might promote escape from therapy-induced senescence. This review seeks to integrate the current knowledge on survivin's diverse and complex biological functions. By linking the 'old' facts about survivin with recent findings in research areas such as DNA damage response and aging, we want to highlight survivin's crucial role in a variety of cellular processes.

Senescence and apoptosis: dueling or complementary cell fates?

In response to a variety of stresses, mammalian cells undergo a persistent proliferative arrest known as cellular senescence. Many senescence-inducing stressors are potentially oncogenic, strengthening the notion that senescence evolved alongside apoptosis to suppress tumorigenesis. In contrast to apoptosis, senescent cells are stably viable and have the potential to influence neighboring cells through secreted soluble factors, which are collectively known as the senescence-associated secretory phenotype (SASP). However, the SASP has been associated with structural and functional tissue and organ deterioration and may even have tumor-promoting effects, raising the interesting evolutionary question of why apoptosis failed to outcompete senescence as a superior cell fate option. Here, we discuss the advantages that the senescence program may have over apoptosis as a tumor protective mechanism, as well as non-neoplastic functions that may have contributed to its evolution. We also review emerging evidence for the idea that senescent cells are present transiently early in life and are largely beneficial for development, regeneration and homeostasis, and only in advanced age do senescent cells accumulate to an organism's detriment.

Ionizing radiation-induced responses in human cells with differing TP53 status

Ionizing radiation triggers diverse responses in human cells encompassing apoptosis, necrosis, stress-induced premature senescence (SIPS), autophagy, and endopolyploidy (e.g., multinucleation). Most of these responses result in loss of colony-forming ability in the clonogenic survival assay. However, not all modes of so-called clonogenic cell "death" are necessarily advantageous for therapeutic outcome in cancer radiotherapy. For example, the crosstalk between SIPS and autophagy is considered to influence the capacity of the tumor cells to maintain a prolonged state of growth inhibition that unfortunately can be succeeded by tumor regrowth and disease recurrence. Likewise, endopolyploid giant cells are able to segregate into near diploid descendants that continue mitotic activities. Herein we review the current knowledge on the roles that the p53 and p21(WAF1) tumor suppressors play in determining the fate of human fibroblasts (normal and Li-Fraumeni syndrome) and solid tumor-derived cells after exposure to ionizing radiation. In addition, we discuss the important role of WIP1, a p53-regulated oncogene, in the temporal regulation of the DNA damage response and its contribution to p53 dynamics post-irradiation. This article highlights the complexity of the DNA damage response and provides an impetus for rethinking the nature of cancer cell resistance to therapeutic agents.

Beclin 1 interactome controls the crosstalk between apoptosis, autophagy and inflammasome activation: impact on the aging process

Autophagy and apoptosis are crucial cellular housekeeping and tissue survival mechanisms. There is emerging evidence of important crosstalk between apoptosis and autophagy which can be linked to inflammasome activation. Beclin 1 is a platform protein which assembles an interactome consisting of diverse proteins which control the initiation of autophagocytosis and distinct phases in endocytosis. Recent studies have demonstrated that the anti-apoptotic Bcl-2 family members can interact with Beclin 1 and inhibit autophagy. Consequently, impaired autophagy can trigger inflammasome activation. Interestingly, the hallmarks of the ageing process include a decline in autophagy, increased resistance to apoptosis and a low-grade inflammatory phenotype. Age-related stresses, e.g. genotoxic, metabolic and environmental insults, enhance the expression of NF-κB-driven anti-apoptotic Bcl-2 proteins which repress the Beclin 1-dependent autophagy. Suppression of autophagocytosis provokes inflammation including NF-κB activation which further potentiates anti-apoptotic defence. In a context-dependent manner, this feedback defence mechanism can enhance the aging process or provoke tumorigenesis or cellular senescence. We will review the role of Beclin 1 interactome in the crosstalk between apoptosis, autophagy and inflammasomes emphasizing that disturbances in Beclin 1-dependent autophagy can have a crucial impact on the aging process.

Role of cellular senescence in hepatic wound healing and carcinogenesis

A state of permanent growth arrest characterises a senescent cell. Both the beneficial and deleterious effects that have accrued in senescent cells are observed in a complex organ, such as the liver. Injury to liver tissues triggers processes of regeneration and associated wound healing. Persistent injury can also lead to the neoplastic state. Recent evidence linked the senescent characteristics of the cells to the beneficial processes of wound healing and tumour surveillance in the liver. On the other hand, the secretory phenotype of senescent cells can also selectively promote undesirable neoplastic progression. In an evolutionary context, a senescent cell can function primarily as an adaptive response featuring the characteristics of altruism, trade-offs and bystander effects. Using the liver cell as a model system, this review focuses on the current knowledge of the role of senescence in these seemingly contradictory cell phenomena.

Context-Dependent Regulation of Autophagy by IKK-NF-κB Signaling: Impact on the Aging Process

The NF-κB signaling system and the autophagic degradation pathway are crucial cellular survival mechanisms, both being well conserved during evolution. Emerging studies have indicated that the IKK/NF-κB signaling axis regulates autophagy in a context-dependent manner. IKK complex and NF-κB can enhance the expression of Beclin 1 and other autophagy-related proteins and stimulate autophagy whereas as a feedback response, autophagy can degrade IKK components. Moreover, NF-κB signaling activates the expression of autophagy inhibitors (e.g., A20 and Bcl-2/xL) and represses the activators of autophagy (BNIP3, JNK1, and ROS). Several studies have indicated that NF-κB signaling is enhanced both during aging and cellular senescence, inducing a proinflammatory phenotype. The aging process is also associated with a decline in autophagic degradation. It seems that the activity of Beclin 1 initiation complex could be impaired with aging, since the expression of Beclin 1 decreases as does the activity of type III PI3K. On the other hand, the expression of inhibitory Bcl-2/xL proteins increases with aging. We will review the recent literature on the control mechanisms of autophagy through IKK/NF-κB signaling and emphasize that NF-κB signaling could be a potent repressor of autophagy with ageing.

Cellular Senescence - its role in cancer and the response to ionizing radiation

Cellular senescence is a normal biological process that is initiated in response to a range of intrinsic and extrinsic factors that functions to remove irreparable damage and therefore potentially harmful cells, from the proliferative pool. Senescence can therefore be thought of in beneficial terms as a tumour suppressor. In contrast to this, there is a growing body of evidence suggesting that senescence is also associated with the disruption of the tissue microenvironment and development of a pro-oncogenic environment, principally via the secretion of senescence-associated pro-inflammatory factors. The fraction of cells in a senescent state is known to increase with cellular age and from exposure to various stressors including ionising radiation therefore, the implications of the detrimental effects of the senescent phenotype are important to understand within the context of the increasing human exposure to ionising radiation. This review will discuss what is currently understood about senescence, highlighting possible associations between senescence and cancer and, how exposure to ionising radiation may modify this.

Apoptosis and aging: increased resistance to apoptosis enhances the aging process

Apoptosis is a vital component in the evolutionarily conserved host defense system. Apoptosis is the guardian of tissue integrity by removing unfit and injured cells without evoking inflammation. However, apoptosis seems to be a double-edged sword since during low-level chronic stress, such as in aging, increased resistance to apoptosis can lead to the survival of functionally deficient, post-mitotic cells with damaged housekeeping functions. Senescent cells are remarkably resistant to apoptosis, and several studies indicate that host defense mechanisms can enhance anti-apoptotic signaling, which subsequently induces a senescent, pro-inflammatory phenotype during the aging process. At the molecular level, age-related resistance to apoptosis involves (1) functional deficiency in p53 network, (2) increased activity in the NF-κB-IAP/JNK axis, and (3) changes in molecular chaperones, microRNAs, and epigenetic regulation. We will discuss the molecular basis of age-related resistance to apoptosis and emphasize that increased resistance could enhance the aging process.

The role of Fas ligand protein in the oxidative stress induced by azoxymethane on crypt colon of rats

PURPOSE: To study the protein Fas ligand (FasL) on the expression of apoptosis, using a model of oxidative stress induced by azoxymethane (AOM), in the crypt of colon in rats. METHODS: Wistar rats (n=14) were assigned into two groups: control (n=7) and AOM (n=7). A single subcutaneous administration of AOM (5mg/kg) or saline solution was performed at the beginning of third week and after three hours samples of proximal colon were collected. The expression of FasL was quantified (Software ImageLab) in percentage of areas in the top, base and all crypt. Results were expressed as mean ± sd (Shapiro-Wilks test and t Student test) (p < 0.05). RESULTS: In the animals of CG there was no significant difference between the FasL expression of the top (10.75±3.33) and basal (11.14±3.53) colon crypt (p=0.34293740). In the animals of AOM there was no significant difference between the FasL expression of the top (8.86±4.19) and basal (8.99±4.08) colon crypt (p=0.78486003). In the animals of CG (10.95±3.43) and AOM (8.92±4.13) there was a significant difference of the FasL expression (p=0.026466821). A significantly decrease on the FasL expression was observed in the animals of CG (10.75±3.33) and AOM (8.86±4.19) in the top crypt (p=0.00003755*). A significant decrease was also observed in the animals of CG (11.14±3.53) and AOM (8.99±4.08) in the basal colon crypt (p=0.00000381**). CONCLUSION: Azoxymethane induce the oxidative stress and the significantly decrease of FasL expression, although there is no significant difference between basal and top of colon crypt linked to consumption-activation of Fas ligand.

Human telomeres are hypersensitive to UV-induced DNA Damage and refractory to repair

Telomeric repeats preserve genome integrity by stabilizing chromosomes, a function that appears to be important for both cancer and aging. In view of this critical role in genomic integrity, the telomere's own integrity should be of paramount importance to the cell. Ultraviolet light (UV), the preeminent risk factor in skin cancer development, induces mainly cyclobutane pyrimidine dimers (CPD) which are both mutagenic and lethal. The human telomeric repeat unit (5′TTAGGG/CCCTAA3′) is nearly optimal for acquiring UV-induced CPD, which form at dipyrimidine sites. We developed a ChIP–based technique, immunoprecipitation of DNA damage (IPoD), to simultaneously study DNA damage and repair in the telomere and in the coding regions of p53, 28S rDNA, and mitochondrial DNA. We find that human telomeres in vivo are 7-fold hypersensitive to UV-induced DNA damage. In double-stranded oligonucleotides, this hypersensitivity is a property of both telomeric and non-telomeric repeats; in a series of telomeric repeat oligonucleotides, a phase change conferring UV-sensitivity occurs above 4 repeats. Furthermore, CPD removal in the telomere is almost absent, matching the rate in mitochondria known to lack nucleotide excision repair. Cells containing persistent high levels of telomeric CPDs nevertheless proliferate, and chronic UV irradiation of cells does not accelerate telomere shortening. Telomeres are therefore unique in at least three respects: their biophysical UV sensitivity, their prevention of excision repair, and their tolerance of unrepaired lesions. Utilizing a lesion-tolerance strategy rather than repair would prevent double-strand breaks at closely-opposed excision repair sites on opposite strands of a damage-hypersensitive repeat.

Telomeres consist of a repeated sequence located at each end of each chromosome. This repeated sequence is required for chromosomal stability and integrity, a function important for both cancer and aging. The DNA sequence of human telomeres is 5–10 kb of a repeated double-strand hexamer (5′TTAGGG/5′CCCTAA). In theory, this sequence is nearly optimal for acquiring UV-induced DNA damage. We developed a novel technique, the immunoprecipitation of DNA damage (IPoD), to study DNA damage induction and repair in the telomere and in coding regions (p53, 28S rDNA, and mitochondrial DNA). We find that human telomeres are hypersensitive to UV-induced DNA photoproducts and that the removal of those DNA photoproducts is almost absent. Cells containing persistent high levels of telomeric DNA damage nevertheless proliferate and chronic UV irradiation of cells does not accelerate telomere shortening. Telomeres are therefore unique in at least three respects: their biophysical UV sensitivity, their prevention of excision repair, and their tolerance of unrepaired lesions.

Alterations in microRNA expression in stress-induced cellular senescence

We investigated miRNA expression changes associated with stress-induced premature senescence (SIPS) in primary cultures of human diploid fibroblast (HDF) and human trabecular meshwork (HTM) cells. Twenty-five miRNAs were identified by miRNA microarray analysis and their changes in expression were validated by TaqMan real-time RT-PCR in three independent cell lines of HTM and HDF. SIPS in both HTM and HDF cell types was associated with significant down-regulation of four members of the miR-15 family and five miRNAs of the miR-106b family located in the oncogenic clusters miR-17-92, miR-106a-363, and miR-106b-25. SIPS was also associated with up-regulation of two miRNAs (182 and 183) from the miR-183-96-182 cluster. Transfection with miR-106a agomir inhibited the up-regulation of p21(CDKN1A) associated with SIPS while transfection with miR-106a antagomir led to increased p21(CDKN1A) expression in senescent cells. In addition, we identified retinoic acid receptor gamma (RARG) as a target of miR-182 and showed that this protein was down-regulated during SIPS in HDF and HTM cells. These results suggest that changes in miRNA expression might contribute to phenotypic alterations of senescent cells by modulating the expression of key regulatory proteins such as p21(CDKN1A) as well as by targeting genes that are down-regulated in senescent cells such as RARG.

CARF Is a vital dual regulator of cellular senescence and apoptosis

The tumor suppressor protein, p53, is central to the pathways that monitor the stress, DNA damage repair, cell cycle, aging, and cancer. Highly complex p53 networks involving its upstream sensors and regulators, downstream effectors and regulatory feedback loops have been identified. CARF (Collaborator of ARF) was shown to enhance ARF-dependent and -independent wild-type p53 function. Here we report that (i) CARF overexpression causes premature senescence of human fibroblasts, (ii) it is vital for replicative and stress-induced senescence, and (iii) the lack of CARF function causes aneuploidy and apoptosis. We provide evidence that CARF plays a dual role in regulating p53-mediated senescence and apoptosis, the two major tumor suppressor mechanisms.